Surface treatment of ferrous metals



EXAWNEFE CROSS RZFZEZEECE E. G. WEATHERLEY ETAL SURFACE TREA'IHENT 0F FERROUS METALS Filed Jan. 15, 1960 March 26, 1963 United States Patent 3,083,122 SURFACE TREATMENT OF FERROUS METALS Eric George Weatherley, London, and Clifford Garnett,

Sunbury-on-Thames, England, assignors to Metal Diffusions Limited, Isleworth, England, a company of Great Britain Filed Jan. 15, 1960, Ser. No. 2,679 Claims priority, application Great Britain Jan. 19, 1959 29 Claims. (Cl. 117-107) The invention relates to the surface treatment of metals and more particularly to methods and means whereby the surface of a ferrous metal article is coated and/or alloyed with another metal, to increase its resistance to corrosion, oxidation or wear, by a diffusion process.

In carrying out such a process, for example, a chromising process, the ferrous article, for example, a mild steel article or a plane sheet, rod or block of mild steel, is generally contained in a sealed container in an atmosphere comprising a suitable compound of the metal which is to form the coating layer. The diffusion operation is effected at elevated temperature, for example, a temperature in the range 8001200 C., and diffusion of the coating metal into the surface layers of the ferrous article occurs to a greater or lesser extent.

It is an object of the invention to provide a composition for metallising, for example, chromising a ferrous article, which will have a long life and which will assist in obtaining diffusion to a greater depth of the coating metal on the ferrous article.

It is a further object of the invention to provide a metallising composition which is effective to remove, or lessen the concentration of, iron halides from the sphere of diffusion.

It is an other object of the invention to provide a metallising composition which permits higher concentrations of the diffusing or coating metal to be obtained at the surface of the ferrous article.

A further object of the invention is to provide a sealing composition which mitigates or avoids loss of the diffusing or coating metal from the vessel or box in which the diffusion is carried out. Loss of the diffusing or coating metal in this manner is fairly pronounced with known sealing compositions.

It is another object of the invention to provide a sealing composition which may be poured as a liquid into the joints to be sealed before heating of the diffusion box or container to the diffusion temperature, is commenced.

According to the invention, a method of coating a ferrous article with another metal by diffusion comprises carrying out the diffusion with a halide of the coating metal in the presence of a substance, preferably ammonium nitrate, effective to remove iron halides from the gaseous phase during diffusion.

According to the invention furthermore, there is provided a metallising composition comprising a sand of high purity and of a particle size to permit gaseous diffusion, and a nitrate effective to remove ferrous halides from the sphere of the diffusion reaction on the surface of a ferrous article during diffusion of the ferrous article with the metallising composition.

According to the invention moreover, the metallising composition for use in the coating of the surface of a ferrous article with another metal by a diffusion process, may comprise a minor proportion, preferably not less than 5% by weight of the composition, of a halide, preferably a higher halide, of the metal to be diffused and a sand of a high degree of purity.

The degree of purity of the sand should be such as to provide a surface mass which is capable of adsorbing 3,083,122 Patented Mar. 26, 1963 the metal of the halide without any substantial amount of the metal entering into chemical combination with the sand. The sand preferably contains not less than 99.3% silicon dioxide and not more than 0.05% iron estimated as ferric oxide. The particle size of the sand must be such as to permit gaseous diffusion through its mass. Advantageously not less than 40% and preferably not less than 70% of the sand is of a particle size within the range 72-100 B.S.S. and advantageously not more than 5% and preferably not more than 1% of the sand is larger than 16 B.S.S. The preferred coating metal halide in the composition is a fluoride.

The metallising composition is particularly suitable for use in chromising in which case the metal halide in the composition is advantageously a chromic halide, preferably chromic fluoride. The composition may, however, be used for diffusing another metal, for example, vanadium, manganese, niobium, tantalum, tungsten or aluminium, into the surface layers of the ferrous article, in which case a corresponding halide of that other metal is the metal halide forming part of the metallising composition. The composition may also be used for diffusing two or more metals simultaneuosly in which case the corresponding halides of such metals form part of the metallising composition.

In a modification according to the invention, the metallising composition advantageously includes a small proportion of an additive effective to reduce the time necessary to complete the diffusing or metallising operation, for example, ammonium chloride, ammonium bromide or ammonium iodide, the preferred additive being ammonium chloride. This additive is preferably present in an amount of not less than 0.01% by weight of the composition.

When the metal halide in the metallising composition is a fluoride, the composition advantageously includes a small amount of a substance which evolves hydrogen fluoride at the temperature of the diffusing or metallising operation. A suitable compound for this purpose is ammonium fluoride, and when used it is preferably present in an amount of not less than 0.1% by weight of the metal halide/sand composition.

The ammonium nitrate dissociates with the evolution of nitrous oxide at the temperature of the metallising or diffusion process. The presence of the nitrate in the metallising composition removes iron halides from the gaseous phase and assists in maintaining a vapour pressure favourable for chromium halides and other coating metal halides.

An example of the metallising composition according to the invention is 10% by weight of chromic fluoride by weight of sand.

When the additives hereinbefore referred to are incorpoample of the composition would then be:

10% by weight of chromic fluoride 89.39% by weight of sand 0.5% by weight of ammonium fluoride 0.1% by weight of ammonium chloride 0.01% by weight of ammonium nitrate When the metallisting composition incorporates the ammonium salts, the ferrous article to be treated may be buried in the composition and the whole may then be raised to an elevated temperature, for example 1,000 C., which is effective to promote diffusion of the metal of the metal halide into the surface layers of the ferrous article being treated.

According to the invention furthermore, in the metallising composition for coating the surface of the ferrous article with another metal by a diffusion process, the other metal may be present in finely divided form in the elementary state in admixture with a sand of the kind hereinbefore specified.

The proportion of the finely-divided metal in the composition is preferably about 4%-8% by weight and is not less than 2%-3% by weight. The metallising composition is particularly adapted for use in a chromising operation in which case the metal in the composition is finely-divided chromium. However, the chromium may be replaced by other metals, for example vanadium, manganese, niobium, tantalum, tungsten or aluminium. Mixtures of two or more of the metals may also be used.

According to the invention moreover, a process for coating the surface of a ferrous article by a diffusion method comprises coating the surface of the article with an aqueous solution of ammonium nitrate and of one or more of the additives ammonium fluoride, ammonium chloride, ammonium bromide and ammonium iodide and thereafter surrounding the ferrous article, preferably closely, with the metal/sand mixture or metal halide/ sand mixture hereinbefore described, and then raising the whole to a temperature which is effective to promote diffusion of the metal from the metal/sand mixture, or of the metal of the metal halide in the case of the metal halide/ sand mixture, into the surface layers of the ferrous article. In this process, the concentration of the aqueous solution is preferably such as to ensure that after coating the article with the solution, the ammonium salts are present in an amount such as to give the results hereinbefore described in relation to the additives.

According to the invention furthermore, a process for introducing another metal into the surface layers of a metal article by a diffusion method comprises coating the surface of the ferrous article with a suspension, solution or dispersion in an aqueous or organic medium of a halide of that other metal and a minor proportion of ammonium nitrate with or without one or more ammonium halides. Preferably, an aqueous medium is employed, particularly one which contains a surface active agent. The preferred surface active agents are non-ionic surface active agents, for example, glycols and fatty acid esters of polymerised and unpolymerised glycols, such as triethylene glycol monostearate, glycol monostearate and glycol monolaurate. When an organic medium is used as the solvent or continuous phase, it is preferably a long chain aliphatic amine, such as one of those available under the trade name Casa.

The coating of the ferrous articles with the solution or dispersion may be effected by dipping or spraying, after which the dipped or sprayed article is preferably airdried. The article is then heated in a sealed container and raised to a temperature adequate to promote the diffusion of the metal from the coating of the metal halide into the surface layers of the ferrous article.

This feature of the invention is particularly adapted for use in chromising operations in which case the metal halide is a chromium halide, the preferred halide being chromic fluoride. The method is not, however, limited to the use of chromium halides but may also be employed with the corresponding halides of other metals, for example, vanadium, tungsten, tantalum, aluminium, manganese or niobium.

According to a further modification of the invention, a suitable carrier, for example, asbestos cloth, glass wool, glass mat or a cellulosic material such as paper, is impregnated with the solution or dispersion hereinbefore referred to and the ferrous article to be treated is enveloped in or covered by the impregnated carrier, after which the ferrous article and the impregnated carrier are brought to a temperature adequate to effect diffusion of the metal of the halide into the surface layers of the article being treated. It will be understood that the carrier must be impregnated with sufiicient of the metal halide to permit the desired amount of the metal to be diffused into the surface layers of the article being treated.

A number of articles can be suitably treated at the same time by interleaving the articles with sheets or layers of the impregnated carrier.

The carrier is preferably virtually free from sulphur, since it has been found that the presence of sulphur hinders the diffusion of the metal into the surface layers of the articles being treated. If a relatively large proportion of sulphur is present, diffusion of the metal into the ferrous article may be substantially completely prevented. Little, if any sulphur is normally present in glass 'wool or asbestos cloth, but deleterious amounts of sulphur are frequently present in certain types of paper, card-board and the like.

The process of the invention in which a solution or dispersion of the active compound is used, has the great advantage that higher rates of production are readily obtainable since the necessity of removing the treated article from the blanket or mass of the active mix does not then arise.

In the process according to the invention, the metallising composition should contain sufiicient of the chromium or other metal being diffused to provide the article being treated with not less than 0.01 gram of chromium, or the equivalent of other metal, per square centimetre of surface. Preferably, the metal is present in the metallising composition in an amount sufficient to provide a diffused coating on the ferrous article of between 0.01 gram and 0.15 gram of chromium (or the equivalent in the case of another metal) per square centimetre of surface area of the ferrous article, particularly when the article is of mild steel.

According to the invention moreover, there is provided a composition for sealing the box or vessel in which the diffusion or coating of the ferrous article is carried out, the sealing composition comprising a mixture or composition which is liquid at the temperature at which the metallising or diffusion process is to be effected, the composition being substantially saturated with respect to the metal, for example, chromium which is to be diffused. The mixture or composition may, at normal or atmospheric temperature, be a liquid, a gelatinous or syruplike mass, or a solid.

According to the invention furthermore, the sealing composition comprises a mixture which is a solid, a liquid or a gelatinous or syrup-like mass at normal temperature and which, upon being heated to the temperature at which the diffusion process is effected forms a liquid glass or glass-like composition which is substantially saturated with respect to the metal being diffused.

The sealing composition, being substantially saturated with respect to the metal being diffused, cannot absorb more of the metal or compound of that metal from the container in which the metallising or diffusion process is being effected. In this manner, the disadvantage inherent in the known sealing compositions is reduced since that disadvantage arises, at least in part, from absorption of the metal or metal compound by the sealing composition.

The sealing composition of the invention is particularly adapted for use in a chromising process, in which case the composition is substantially saturated with respect to chromium. The sealing composition is, however, adapted for use with any other metal which may be diffused in such a process, for example, vanadium, manganese, niobium, tantalum, aluminium or tungsten.

The melting point of the composition may readily be adjusted by varying the nature or proportion of its alkali content. Thus, where the composition is to be used in a chromising process and the composition is, in consequence, substantially saturated with chromium, the melting point of the composition may be adjusted to a temperature within the approximate range 800l400 C. by varying the alkali content of the composition. It is preferred that the composition should be such that it is in the liquid phase at a temperature about degrees C.

or more below the temperature at which the difiusion process is to be effected.

The sealing composition may, for example, be disposed in a channel formed in the rim of the body of the container in which the diffusion process is to be effected, the lid or closure of the container being provided with a lip or flange which extends into the channel.

A sealing composition which is solid at atmospheric temperature may be provided as a mixture of ingredients, for example a mixture of sand, sodium carbonate and powdered chromium, which at the temperature at which the process is to be carried out fuses to form a liquid glass which is saturated with the metal being diffused, this metal being chromium in the particular mixture given by way of illustration. Upon cooling, the sealing composition solidifies to a glass and the solid glass may then be broken and/or powdered and may be re-used as a sealing composition.

When the sealing composition is prepared in situ from a mixture of solid ingredients such as that hereinbefore specified, the channel and the mixture are preferably first wetted with water. If this is not done, then undue loss of sodium carbonate or other equivalent salt may occur as air is expelled from the container when its temperature is raised, the salt being removed from the channel by entrainment in the expelled air. By wetting the channel and the mixture, the sodium carbonate forms the mono or other hydrate which is retained in the channel during the heating-up period. Alternatively, the original mixture of ingredients placed in the channel may contain such an excess of sodium carbonate or other equivalent salt as will compensate or allow for the loss of the salt which is entrained in the air expelled from the container during the heating-up period.

The metal which is incorporated in such a mixture of ingredients is preferably in finely-divided form and its particle size is advantageously less than 200 E58.

A sealing composition which is liquid or is a syrup-like mass at atmospheric temperature comprises waterglass which contains an amount of the metal to be diffused such that the sealing composition is substantially saturated with respect to that metal at the temperature at which the diffusion is to be carried out.

The term waterglass is to be understood as comprising one or more alkali-metal silicates and is not limited to sodium silicates.

The silicates in waterglass can be represented by the formula Me O.nSiO in which Me denotes one or more alkali-metals and n is the molecular ratio of silica to alkali. n may be a whole number thus denoting one of a series of silicates, for example, Na SiO Na Si O Na Si O or Na Si O but is more generally a fraction having a value between 1.2 and 4.

The commercially available "grades of waterglass are generally either true solutions or colloidal solutions or suspensions. The following table gives particulars of three main types of commercially available kinds of waterglass.

S.G. Viscosity SiOz NaaO H2O Percent,

20 C. solids Such a solution may, for example, be prepared by admixing sodium dichromate and sodium hydroxide with a suitable commercial waterglass, and when the solution is heated from atmospheric temperature to the temperature at which the diffusion process is to be effected there will generally always be a liquid phase present. Thus, for example, at atmospheric temperature the sodium dichromate and sodium hydroxide are in solution in the waterglass, the waterglass itself being a true or colloidal solution of one or more sodium silicates in water. At C. part at least of the solvent (water) evaporates leaving some solids and a liquid phase consisting of or comprising compounds having formulae be tween Na Si O and Na Si O This liquid phase persists up to about 400 C., above which temperature there is a temperature range in which this liquid becomes a solid. At about 318 C., however, the sodium hydroxide which has been precipitated as a solid, becomes a liquid and forms a continuous liquid phase as the temperature is raised until the solids formed at about 400 C. melt. Such a composition forms a liquid seal during the whole of the heating-up period, that is, from room temperature to the diffusion temperature and persists as a liquid seal at the diffusion temperature.

The waterglass composition containing sodium dichromate hereinbefore referred to, will be used in a diffusion process in which the metal being diffused is chromium, and the amount of dichromate present is such that the composition is substantially saturated with chromium at the temperature at which the diffusion process is carried out. The chromium may, however, be in any suitable form other than as a dichromate. When the metal being diffused is other than chromium, then that other metal will be incorporated in a suitable form in the waterglass composition, the amount of such other metal present being also sufficient substantially to saturate the composition with that metal at the temperature at which diffusion is carried out.

The sealing composition provided according to the invention permits the amount in the container of the metal to be diffused to be appreciably less than that required with known sealing compositions. Known chromising compositions generally contain about 25% by weight of chromium metal, but with the sealing composition of the invention the chromium content of the chromising composition may be very much less, and may even be as low as 3%. Thus, for example, the use of the sealing composition in a chromising process permits the chromium in the container to be reduced to about of the amount which it is desired to diffuse into the metal article.

When the sealing composition is used in a process in which two or more metals are being diffused simultaneously, the sealing composition is substantially saturated with respect to each of such metals.

The invention further provides a process for increasing the resistance to corrosion or oxidation of an article metallised by a diffusion method, which process comprises coating the metallised article with an aluminium halide, preferably aluminium chloride and heating the article to an elevated temperature such as to ensure that aluminium diffuses into and/or alloys with the surface layer of the article. Such temperature will, in general, be within the range 700 C.-1050 C. This process according to the invention is particularly adapted for improving the properties of the surface coating of a chromised article. The process may, however, be applied to any article whether metallised or not, provided that the surface layers of the article comprise a metal which is capable of forming an alloy with aluminium. The process has the substantial advantage that the article, particularly in the case of a chromised article, requires to be maintained at the requisite elevated temperature for only a relatively short time.

Advantageously the metal article to be treated is coated with the aluminium halide preferably aluminium chloride,

7 by a dipping operation. The dipping solution is preferably a saturated aqueous solution of the halide. The aluminium halide may, however, be applied to metal articles in another form, for example, as an emulsion.

According to the invention moreover, a ferrous article to be metallised by a diflusion process is pretreated with a halogen acid, preferably a dilute solution of the acid. The preferred acid is hydrochloric acid. The strength of the dilute solution is advantageously about of that of the concentrated acid commercially available. Thus, for example, in the case of hydrochloric acid the solution may contain 3%-4% by weight of hydrogen chloride. However, the concentration of the acid may be lower or higher, for example, from 5% of the strength of the concentrated acid up to the concentrated acid itself. The main consideration is that the acid should attack and be of a strength sufficient to clean the surface of the metal article prior to the diffusion process.

This method of pretreatment according to the invention has been found to lead to deeper penetration and heavier concentration of the diffused metal in the article. This is particularly true in the case where the metal to be diffused into the surface layers of the article is chromium but the pretreatment is not limited to a chromising operation but may be also carried out where the metal to be diffused is other than chromium, for example, vanadium, manganese, aluminium, tungsten, tantalum or niobium.

The acid solution is advantageously used hot and it preferably contains a wetting agent. The wetting agent is preferably free from sulphur and is advantageously a nitrogen compound other than a quaternary ammonium compound.

One example of a box or container suitable for carrying out the coating of a ferrous article by the diffusion process is diagrammatically illustrated in vertical section in the accompanying drawing.

The box comprises a casing 1 of rectangular section provided on its inner surface with an integral member 2 of angle section which serves to define a channel 3 extending around the whole of the inner surface of the easing at the upper end of the box. A lid 4 rests on the upper, outer end of the member 2 and the lid is provided with an endless, depending flange or lip 5 which extends into the channel 3. All parts of the box and lid are of mild steel.

In use, the box may be filled with a metallising composition 7 in which the ferrous articles 6 to be coated are buried. An appropriate sealing composition 8 of the kind hereinafter illustrated in any one of Examples 1 to 11 is then poured or placed in the channel 3 and the lid 4 is placed in position with its depending lip or flange 5 dipping into the sealing composition to seal the box. It will be understood that the sealing composition 8 is such that it is liquid or sufficiently soft to flow at the temperature at which the diffusion process is to be carried out, and that it is substantially saturated with respect to the coating metal or metals. Each coating metal is present as such or in the form of a halide or halides in the mass of the metallising composition, and/or is present as a film of one or more halides on the articles 6 themselves.

The invention is illustrated in the following examples.

Two sands A and B were employed in the examples, the analyses of the sands being as follows:

8 Particle size:

82% by weight within the range 72-100 B.S.S. 0.44% by weight greater than 16 B.S.S.

SAND B Percent by weight Si0 99.56 A1 0 0.22 Fe O 0.02 CaO 0.11 MgO 0.05 Na O/K' O 0.02 Loss on ignition 0.02

Particle size:

84.7% by weight within the range 72-100 B.S.S. 0.1% by weight greater than 16 B.S.S.

Example 1 A metallising composition was made by intimately mixing together 10.00 parts by weight of niobium fluoride 89.19 parts by weight of sand A 0.70 parts by weight of ammonium fluoride 0.10 parts by weight of ammonium chloride 0.01 parts by weight of ammonium nitrate A part of the composition was placed in a box of the same form as that shown in the accompanying drawing. A piece of ENZA mild steel sheet was buried in the composition in the box, and the box was then filled level with the lip of the channel member 2 with a further quantity of the composition and the lid 4 was placed on top with its projecting lip 5 extending into the channel 3.

7 parts by weight of finely divided niobium metal (particle size less than 200 B.S.S.) was mixed with a parts by weight of sodium tetrasilicate (Na Si O dissolved in 75 parts by weight of water, and the mixture was poured into the channel 3 of the box to form the seal 8. The sealed box was then placed in a furnace held at a temperature of 1015 C. After the box and its contents had reached this temperature, they were maintained in the furnace at this temperature for a further four hours. The box was then removed from the furnace and allowed to cool to room temperature.

The box was opened and the piece of ENZA mild steel was removed for examination. The composition was found to be free-flowing and it had not sintered or aggregated.

Microscopic examination of the edge of the piece of mild steel, after grinding. polishing and etching. revealed that niobium had difiused 0.003 inch into the mild steel.

The seal 8 was liquid and substantially saturated with respect to niobium at the diffusion temperature of 1015 C. Consequently little or no niobium was lost from the box by absorption in or combination with the sealing material.

Example 2 9.00 parts by weight of manganese fluoride 90.28 parts by weight of sand A 0.50 parts by weight of ammonium fluoride 0.20 parts by weight of ammonium chloride 0.02 parts by weight of ammonium nitrate were intimately mixed together and part of the mixture or metallising composition was placed in a box of the same construction as that shown in the accompanying drawing. A grey cast iron moulding insert was buried in the composition in the box, and the box was then filled to the level of the lip of channel member 2 with a further quantity of the composition. The lid 4 was placed in position with its projecting lip 5 extending into the channel 3.

A mixture of 100 parts of sodium tetrasilicate z i s) parts of sodium permanganate (NaMnO and 75 parts of water (all parts being by weight) were made into a solution and the solution was poured into the channel 3 to form the liquid seal 8. The sealed box was then placed in a furnace held at a temperature of 900 C. After allowance had been made for the box and its contents to reach this temperature, the box and its contents were then held in the furnace at 900 C. for four hours. The box was then removed from the furnace and allowed to cool to room temperature.

The box was opened and the piece of cast iron was removed for examination. The composition was found to be free-flowing and it had not sintered or aggregated.

Microscopic examination of the edge of the piece of cast iron after grinding, polishing and etching, revealed that manganese had diffused 0.0025 inch into the cast iron.

The liquid seal 8 was such that it was substantially saturated with manganese at the temperature of 900 C. used in the ditfusion process.

Example 3 A metallising composition was made by intimately mixing together .00 parts by weight of tungsten metal powder 94.71 parts by weight of sand B 0.25 parts by weight of ammonium fluoride 0.02 parts by weight of ammonium chloride 0.02 parts by weight of ammonium nitrate Part of the composition was placed into the box illustrated in the accompanying drawing. A piece of ENZA mild steel sheet was buried in the composition in the box, and the box then filled to the level of the lip of channel member 2 with a further amount of the composition and the lid 4 was placed in position with its projecting lip 5 extending into the channel 3.

A mixture of 100 parts of sodium tetrasilicate z e s) and 100 parts of sodium tungstate (Na W O .28H O) were dissolved in 75 parts of water (all parts being by weight) and the solution was poured into the channel 3 and the sealed box was then placed in a furnace held at a temperature of 1015" C. After allowance had been made for the box and the contents to reach this temperature, the box was retained in the furnace at that temperature for a further four hours. The box was then removed from the furnace and allowed to cool to room temperature.

The box was opened and the piece of ENZA mild steel removed for examination. The composition was found to be free-flowing and it had not sintered or aggregated.

Microscopic examination of the edge of the piece of mild steel after grinding, polishing and etching, revealed that tungsten had diffused 0.005 inch into the mild steel.

The seal 8 which was liquid at the dilfusion temperature of 1015 C., was substantially saturated with respect to tungsten at that temperature.

Example 4 A metallising composition was made by intimately admixing together 5.00 parts by weight of chromium metal powder 94.77 parts by weight of sand B 0.20 parts by weight of ammonium fluoride 0.02 parts by weight of ammonium chloride 0.01 parts by weight of ammonium nitrate Part of the composition was placed into the box shown in the accompanying drawing. A grey cast iron moulding insert was buried in the composition in the box, and the box was then filled level with the lip of the channel member 2 with a further quantity of the composition. The lid 4 was placed on top with its projecting lip 5 extending into the channel 3.

A liquid seal of the same composition as that described in Example was poured into the channel 3 and the sealed box was then placed in a furnace held at a temperature of 900 C. After allowance had been made for the box and its contents to reach this temperature, the box was kept in the furnace at that temperature for a further four hours. The box was then removed from the furnace and allowed to cool to room temperature.

The box was opened and the piece of cast iron was removed for examination. The composition was found to be free-flowing and it had neither sintered nor aggregated.

Microscopic examination of the edge of the piece of cast iron after grinding, polishing and etching, revealed that chromium had diffused 0.004 inch into the cast iron.

Example 5 5 parts by weight of tungsten metal powder 95 parts by weight of sand B were intimately mixed together and part of the mixture was placed in a box of the same construction as that shown in the accompanying drawing.

A piece of ENZA mild steel sheet fan blade was dipped into an aqueous solution containing:

15% by weight of ammonium fluoride 1.2% by weight of ammonium chloride 1.2% by weight of ammonium nitrate and allowed to dry in a current of warm air. The piece of mild steel sheet was buried in the mixture of tungsten powder and sand in the box, and the box was filled to the level of the lip of the channel member 2 with a further quantity of the sand/tungsten powder mixture. The lid 4 was placed in position with its projecting lip 5 extending into the channel 3.

A liquid seal of the same composition as that used in Example 3 was poured into the channel 3 and the sealed box placed into a furnace held at a temperature of 1020" C. After allowance had been made for the box and its contents to reach this temperature, the box was maintained at that temperature in the furnace for 3% hours. The box was then removed from the furnace and allowed to cool to room temperature.

The box was opened and the piece of ENZA mild steel removed for examination. No sintering of the sand/ tungsten powder mixture had occurred.

Microscopic examination of the edge of the mild steel after grinding, polishing and etching, revealed that tungsten had diffused 0.0047 inch into the mild steel.

Example 6 10 parts by weight of manganese fluoride and parts by weight of sand B were intimately mixed together and placed in the box shown in the accompanying drawing. A piece of ENZA mild steel sheet fan blade was dipped into an aqueous solution containing:

15% by weight of ammonium fluoride 1.2% by weight of ammonium chloride 1.2% by weight of ammonium nitrate and allowed to dry in a current of warm air. The piece of mild steel sheet was then buried in the manganese fluoride/sand mixture in the box and the box was filled to the level of the lip of channel member 2 with a further quantity of the manganese fluoride/sand mixture. The lid 4 was placed on top with its projecting lip 5 extending into the channel 3. A liquid seal of the same composition as that used in Example 2 was poured into the channel 3 and the sealed box was placed into a furnace held at a temperature of 1020 C. After the box and its contents had reached that temperature, the box was kept in the furnace and at that temperature for a further 3% hours. The box was then removed from the furnace and allowed to cool to room temperature.

1 1 The box was opened and the piece of ENZA mild steel removed for examination; no sintering of the manganese fluoride/sand mixture had occurred.

Microscopic examination of the edge of the mild steel after grinding, polishing and etching, revealed that manganese had difiused 0.006 inch into the mild steel.

Example 7 An aqueous dispersion containing by weight:

25% of chromium fluoride of ammonium fluoride 1% of ammonium chloride 0.5% of ammonium nitrate was prepared and to this was added 0.05% by volume of monoethylene glycol.

The dispersion was kept agitated by mechanical means and sheets of asbestos cloth were soaked in the dispersion, the sheets being cut exactly to the internal dimensions of the base of the box shown in the accompanying drawing.

A sheet of the impregnated asbestos cloth was removed and placed on the bottom of the box and perforated plates of ENZA mild steel forming part of a domestic oil burner unit, were laid on the sheet of impregnated asbestos. A further impregnated asbestos sheet was placed on top of the burner plates and the process repeated until the box was filled to the level of the lip of the channel member 2 with alternate layers of impregnated asbestos sheets and burner plates, the uppermost layer being an impregnated asbestos sheet.

A liquid seal of the same composition as that described in Example 11 was poured into the channel 3 to seal the box. The sealed box was loaded into a furnace held at a temperature of 1020 C. After the box and its contents had reached that temperature, the box was held at that temperature in the furnace for a further six hours. The box was then removed and allowed to cool to room temperature.

When cold, the box was opened and the contents removed and examined. Microscopic examination of a ground, polished, and etched edge of one of the burner plates showed that chromium had diffused 0.0035 inch into the original steel.

Example 8 A mixture of the following ingredients- Parts by weight Sand 75 Sodium carbonate (anhydrous) 44 Chromium metal 22 10.00 parts by weight of chromic fluoride 8939 parts by weights of sand A 0.50 parts by weight of ammonium fluoride 0.10 parts by weight of ammonium chloride 0.01 parts by weight of ammonium nitrate and a mild steel article to be chromised had been buried more or less centrally in the composition in the box.

The closed container was then heated. Virtually no loss of sodium carbonate from the channel occurred as air was expelled from the container during the heating. At 794 C. the powdered mixture in the channel began to sinter and at 820 C. the mixture formed a liquid glass the composition of which was, upon completion of the experiment, found to be 56.6% sio 19.4% Na O 24.0% Cr O This glass is saturated with chromium so that the sealing composition could not absorb further chromium from the atmosphere within the container.

The temperature of the container was raised to 1040 C. at which temperature it was maintained for the period of the diffusion process.

After cooling the container, the solidified vitreous sealing composition was powdered and re-used in a further chromising process which was carried out in substantially the same manner. In this instance, however, there was no need to wet the channel or the sealing composition with water before the powdered sealing composition was placed therein.

Example 9 A powdered, wetted mixture of the following ingredients:

Parts by weight Sand Sodium carbonate (anhydrous) 40 Chromium metal (particle size less than 200 B.S.S.) 21

was placed in the wetted channel 3 of the box shown in the accompanying drawing, and a steel article was chromised in much the same manner as that described in Example 8.

The sealing composition sintered at about 830 C. and formed a glass having a melting point of 880 C. and the following composition:

60% SiO 18% Na O Upon cooling the glass was powdered and re-used as a sealing composition in a further chromising operation.

Example 10 A mixture of the following ingredients:

Parts by weight Sodium tetrasilicate (Na Si O Sodium dichromate (Na Cr O 2H O) 100 Water 75 was made into a solution and placed in the channel 3 of the box shown -in the accompanying drawing. The box was filled with a chromising mixture of the same composition as that used in Example 4, a steel article to be chromised being buried more or less in the middle of the chromising composition. The lid 4 was then placed on the box with its flange 5 dipping into the liquid seal in the channel 3.

The closed box was then heated. At 320 C. the mixture in the channel became completely dehydrated and at 400 C. the mixture began to decompose. As the temperature was further increased, the mixture began to sinter and at a higher temperature it formed a liquid glass the composition of which was, upon completion of the experiment, found to be 46.2% sio 24.1% Na o CI203 This glass is saturated with chromium so that the sealing composition could not absorb further chromium from the atmosphere within the container. The melting point of this glass was found to be 900 C.

The temperature of the container was raised to 1040 C. at which temperature it was maintained for the period of the diifusion process.

was placed in the channel 3 of a box of the same construction as that shown in the accompanying drawing. A steel article was chromised in the box in the same manner as that described in Example 10.

The sealing composition became anhydrous at about 0 320 C. and formed a glass having a melting point of 890 C. Analysis indicated that the composition of the glass was:

44.2% SiO 27.3% N3 0 CI O Example 1 2 A mild steel oil-burner plate which had been chromiumditfused in the manner described in Example 7, was dipped into a saturated aqueous solution of aluminum chloride containing 0.01% of a wetting agent sold under the name Teepol (sodium higher alkyl sulphates), after which the plate was allowed to drain for several minutes. The plate was then suspended for 6 minutes in a furnace maintained at 720 C., removed from the furnace and allowed to cool.

Visual examination of the plate showed that the former bright, chromium-diffused surface was now of a matt blue-grey colour.

Chemical examination and examination under the microscope of the surface layers of the plate showed that aluminium had diffused into the chromium-containing layer and that at the surface of the plate the aluminium chloride had been converted to aluminium oxide. Furthermore, it was found that the aluminium-containing coating was integral with the chromium-diffused material.

The treated burner plate exhibited improved heat resistance over a prolonged period at temperatures above 1040" C. No rusting of the plate occurred after exposure to moist air.

Example 13 An aqueous dispersion containing by weight:

20% of aluminium chloride 2% of ammonium fluoride 2% of ammonium chloride 1% of ammonium nitrate was prepared and to this dispersion was added 0.05% by volume of a surface active agent (sodium monoethylene glycol). A number of bars in EN32 material for use as bath supports in a vitreous enamelling furnace, were dipped into the dispersion and rapidly air dried before being packed together -to fill a box of the kind illustrated in the drawing, and the lid 4 was placed in position.

100 parts of sodium tetrasilicate and 118 parts of sodium aluminate were dissolved in 75 parts of water (all parts being by weight) and the solution was poured into the channel 3 to seal the box. The sealed box was placed in a furnace operating at 790 C. After the box and its contents had attained this temperature, the box was kept in the furnace at the same temperature for a further 2 hours. The box was then removed from the furnace and immediately opened to the air.

On cooling to room temperature, one of the bars was examined microscopically; it was found that a superficial coating of aluminium oxide was formed on the surface and that aluminium had diffused 0.011 inch into the metal.

1 4 Example 14 A piece of chilled grey cast iron was cut into two parts and one part was immersed in 10% w./v. hydrochloric acid at 60 C. for 20 minutes, after which the acidtreated part was rinsed in cold water. The dilute hydrochloric acid contained 0.01% of a wetting agent (Teepo Both of the cast iron parts were then subjected to a process of chromium diffusion in the same vessel so as to ensure that the diffusion process was carried out under precisely the same conditions for both parts. The chromium diffusion process was carried out in the same manner as that described in Example 4. The two parts were thereafter examined. The part which had been pre-treated in dilute hydrochloric acid was found to have a ditfused coating of chromium 0.008 inch deep. The maximum depth of the diffused coating of chromium on the other part was 0.006 inch.

Example 15 A piece of mild steel plate was cut into two parts and one part was immersed in 10% w./v. hydrofluoric acid at 60 C. for 20 minutes, after which the part was rinsed in cold water. The dilute acid contained 0.01% of a wetting agent (Teepol).

Both parts were then subjected in the same box to a process of chromium diffusion in the same manner as that described in Example 4. The part pre-treated with dilute hydrofluoric acid was found to have a diffused coating of chromium 0.005 inch deep, whilst the diffused coating of chromium on the other part was 0.0035 inch deep.

We claim:

1. In a method of coating a ferrous article with another metal by diffusion, the step of carrying out the diffusion at a temperature above 800 C. with a halide of the coating metal in the presence of a major proportion of a solid inert material and a minor proportion of nitrous oxide, the nitrous oxide constituting at least 0.00055% of the combined weight of the said halide, solid inert material and nitrous oxide.

2. In a method of coating a ferrous article with another metal by a diffusion process, the step of carrying out the diffusion at a temperature above 800 C. with a halide of the coating metal in the presence of a major proportion of a sand of high purity and the products of decomposition of a minor proportion of ammonium nitrate, the ammonium nitrate before decomposition during the diffusion process, constituting at least 0.001% of the combined weight of the said halide, sand and ammonium nitrate.

3. A method according to claim 2, in which the halide of the coating metal is generated in the vessel in which the diffusion is carried out.

4. A method according to claim 2, in which the diffusion is carried out in the presence of the products of decomposition of a minor proportion of an ammonium halide, the ammonium halide before decomposition during the diffusion process constituting at least 0.01% of the combined weight of the coating metal halide, sand, ammonium nitrate and ammonium halide.

5. A method according to claim 4, in which the ammom'um halide is a member of the group consisting of ammonium fluoride and ammonium chloride.

6. A method according to claim 4, in which the diffusion is carried out with the ferrous article buried in the said sand, the sand containing not less than 99% by weight of silicon dioxide and not more than 0.1% by weight of iron, estimated as ferric oxide.

7. A method according to claim 6, in which the sand contains not less than 99.3% by weight of silicon dioxide and not more than 0.05% by weight of iron, estimated as ferric oxide.

8. A method according to claim 7, in which not less than 70% of the sand is of a particle size within the range 72-100 British Standard Specification and not more than 15 1% of the sand is greater than 16 British Standard Specification.

9. A method according to claim 8, in which a film comprising said ammonium nitrate and ammonium halide is applied to the ferrous article, the ferrous article being then subjected to diffusion buried in a mixture of the halide of the coating metal and the said sand.

10. A method according to claim 2, in which'the ferrous article is pre-treated with a halogen acid.

11. A method according to claim 2, in which the ferrous article after diffusion of the coating metal into its surface layer is subsequently coated with an aluminium halide and is then heated to an elevated temperature to effect diffusion of aluminium into the surface layer of the ferrous article.

12. A method according to claim 2, in which the diffusion is carried out in a vessel sealed with a composition which is liquid or sufficiently soft to flow at the diffusion temperature and which is substantially saturated with respect to the coating metal.

13. A method according to claim 2, in which the coating metal is a member of the group consisting of chromium, manganese, tungsten, niobium, tantalum and vanadium.

14. A method of coating a number of ferrous articles with another metal, which comprises forming at least two layers of the ferrous articles, separating each layer from an adjacent layer by a flexible inert carrier impregnated with a halide of the coating metal and with ammonium nitrate, and maintaining the layers at an elevated temperature effective to promote diffusion of the coating metal into surface layers of the ferrous articles, the amount of ammonium nitrate in the flexible inert carrier being sufiicient to remove iron halides from the vapour phase during the diffusion process.

15. A metallising composition for a ferrous article, including a member of the group consisting of a metal other than iron and a halide of a metal other than iron, a major proportion of a sand of high purity and of a particle size to permit gaseous difiusion, an ammonium halide in an amount of not less than 0.01% by weight of the composition and ammonium nitrate in an amount of not less than 0.001% by weight of the composition.

16. A metallising composition for coating a ferrous article, comprising a sand containing not less than 99.0% by weight of silicon dioxide and not more than 0.1% by weight of iron, estimated as ferric oxide, ammonium nitrate in an amount of not less than 0.001% by weight of the composition, and a minor proportion of a member of the group consisting of the coating metal in finely divided form and a halide of the coating metal.

17. A composition according to claim 16, in which the halide of the coating metal constitutes not less than 5% by weight of the composition.

18. A composition according to claim 17, in which the halide is fluoride.

19. A composition according to claim 16, in which the coating metal in finely divided form constitutes not less than 3% by weight of the composition, the composition also including at least one ammonium halide.

20. A composition according to claim 19, in which the composition contains not less than 0.1% by weight of ammonium fluoride and not less than 0.01% by weight of ammonium chloride.

21. A composition according to claim 16, in which ammonium nitrate constitutes from 0.005% to 0.1% by weight of the composition.

22. A composition according to claim 16, in which the sand contains not less than 99.3% by weight of silicon dioxide and not more than 0.05% by weight of iron, estimated as ferric oxide.

23. A composition according to claim 22, in which not less than of the sand is of a particle size within the range 72-100 British Standard Specification and not more than 1% is larger than 16 British Standard Specification.

24. A composition according to claim 16, in which the coating metal which is present as a member of the group consisting of coating metal halides and finely divided metal, is chromium.

25. A composition for sealing a vessel in which a metal article is coated with another metal in a dilfusion process, including a substance which is liquid and stable at the temperature at which the difiusion is carried out and which is substantially saturated with respect to the coating metal.

26. A method of sealing a container in which a ferrous article is to be coated with at least one other metal in a diffusion process, which comprises applying to the joints to be sealed a composition which at the diffusion temperature forms a liquid glass which is saturated with respect to each coating metal.

27. A method according to claim 26, in which the composition when applied to the joints at or about room temperature is in liquid form and maintains a liquid phase as its temperature is raised to the temperature at which the diffusion is carried out.

28. In a method of coating a ferrous article with another metal in a diffusion process, the step of carrying out the diffusion process in a container sealed-with a composition comprising a silicate, the composition being liquid at the temperature at which the diffusion is carried out and being substantially saturated with respect to the coating metal.

29. A composition for use in the coating of a ferrous article with another metal by a diffusion process, including a member of the group consisting of the said other metal and a halide of the said other metal, an ammonium halide in an amount of at least 0.01% by weight of the composition, ammonium nitrate in an amount of at least 0.01% by weight of the composition, and at least by weight of the composition, of silicon dioxide in particulate form and of a purity such that it is capable of adsorbing the said other metal at the diffusion temperature without any substantial amount of the said other metal entering into chemical combination with the silicon dioxide.

References Cited in the file of this patent UNITED STATES PATENTS 1,893,782 Marshall Jan. 10, 1933 2,536,774 Samuel Jan. 2, 1951 2,855,332 Samuel Oct. 7, 1958 2,859,132 Novak et a1 Nov. 4, 1958 2,860,098 Fahnoe et al Nov. 11, 1958 

1. IN A METHOD OF COATING A FERROUS ARTICLE WITH ANOTHER METAL BY DIFFUSION, THE STEP OF CARRYING OUT THE DIFFUSION AT A TEMPERATURE ABOVE 900*C. WITH A HALIDE OF THE COATING METAL IN THE PRESENCE OF A MAJOR PROPORTION OF A SOLID INERT MATERIAL AND A MINOR PROPORTION OF NITROUS OXIDE, THE NITROUS OXIDE CONSTITUTING AT LEAST 0.00055% OF THE COMBINED WEIGHT OF THE SAID HALIDE, SOLID INERT MATERIAL AND NITROUS OXIDE. 